The present invention generally relates to turbines and generally relates to surface structures that damp vibration of turbine components.
Operation of a turbine may subject many of the turbine's components to vibrational stresses. Vibrational stresses may shorten the fatigue life of components, thus potentially subjecting them to failure, especially when the components are also subjected to the harsh environment of a gas turbine.
One way to reduce vibrational stresses and extend the life of components may relate to damping the vibration of the component, thus potentially altering vibrational characteristics in such a way to increase its useful life. Mechanical damping mechanisms have been used to damp vibration of turbine components. Examples of the mechanical means include a spring-like damper inserted in a rotor structure beneath the airfoil platform, or a damper included at the airfoil tip shroud.
The phenomenon of damping may generally refer to the process of absorbing and converting the energy associated with a given oscillation into a different form of energy. Damping or energy dissipation can be caused by a combination of mechanisms depending on the mechanical structure (i.e. structural damping) as well as by a variety of mechanisms depending on the material's composition and processing conditions (i.e. material damping). All microscopic and macroscopic mechanisms taking place within the volume of a vibrating part and causing energy dissipation during operation may contribute to material damping.
The removed energy may be converted directly into heat or may be transferred to connected structures or ambient media. Micromechanisms causing internal damping in single or multiphase crystalline metallic materials may be called internal friction. In structural mechanics, it may be common to describe a structure in terms of modal parameters. Each mode corresponds to one degree-of-freedom and is characterized by a resonance frequency, a deformation vector and a modal damping. The structural response for a given force input may then be obtained by linear superposition of all modes. In the context of modal representation, structural damping is expressed in terms of modal damping values.
The desired properties of high strength, stiffness and tolerance to adverse environments appear to be at odds or even incompatible with high internal damping. Viscoelastic materials may show high damping capabilities but may be easily contaminated by their environment and usually must be applied as thick coatings since they unfortunately have insufficient strength properties. Thus the optimization of a damping treatment typically requires not only the proper choice of a damping material, but an understanding of the effects of the geometry of the damping treatment and the modal characteristics of the structure being damped.
Turbine components which might operate at high temperatures (e.g., up to 2500° F.), and/or corrosive/erosive environments, and/or under centrifugal loads must transition through structural resonance conditions to reach their operating envelope. Currently, there are no available adequate damping treatments which survive the turbine environment and do not sacrifice component integrity. U.S. Pat. No. 5,775,049 and U.S. Pat. No. 5,924,261 report Lodengraf materials with low sound speed to damp structural vibration and noise in advanced ship cabinets and electronics enclosures which are filled with granular materials like low density polyethylene beads, or lead shots which are not suitable for gas turbines. U.S. Pat. No. 4,380,574 reports a damping composite where a high damping metal surface layer is deposited on all sides of a poor damping base metal. Examples given are also not suitable for the harsh turbine environments. For example, high damping ferromagnetic alloys or magnetoelastic damping alloys (12Cr steel or Westinghouse's NIVCO10) are prone to fatigue cracking at 600° C. (1100° F.) due to precipitation of brittle intermetallic phases. Moreover, combining metallic base material and high damping surface layer is not ideal where there are large differences in their chemical and physical properties where the intended properties may be negatively affected due to metallurgical events (e.g. diffusional and kinetic processes) during operation. Thus, there may exist a need for good damping properties of surface architectures designed with good mechanical, thermal, and chemical strength.
In certain embodiments, there may be materials (and processes for the manufacture thereof) that possess high material damping under all operating conditions, that have good strength over the entire range of mechanical and thermal stresses, and that facilitate a wide range of construction and/or design of the components.